(1) Field of the Invention
The present invention relates generally to acoustic transducers, and more particularly to acoustic transducers that can generate/detect beams of acoustic energy for a plurality of frequencies.
(2) Description of the Prior Art
Acoustic transducers are devices which generate acoustic energy when excited in a known fashion and/or generate an electrical signal representative of the acoustic energy incident upon the transducer. For example, one prior art single array piezoelectric ceramic transducer 10 is shown in the frontal plan view of FIG. 1 and cross-sectional view of FIG. 2. Transducer 10 includes piezoelectric ceramic material 12 disposed between metallic layers 16a,16b which are deposited on top and bottom surfaces 12a,12b of material 12. Notches, represented by lines 18, are cut in a hatched pattern through metallic layers 16a,16b and into a portion of piezoelectric ceramic material 12 to define an array of pillars 20a,20b capped with metal electrodes 22a,22b formed on surfaces 12a,12b. The surfaces presented by the arrays of electrodes 22a or 22b can serve as the front face plane of transducer 10. Each metal electrode 22a,22b is electrically isolated from adjacent electrodes. The pattern of notches 18 is optimally sized so that the width of each pillar 20a,20b is approximately 0.5.lambda. where .lambda. is the wavelength in the transmission medium of the acoustic energy being generated or received. Metal electrodes 22a are electrically interconnected to one another (not shown for ease of illustration) and connected to electrical lead 24a. In a similar fashion, metal electrodes 22b are electrically interconnected to one another and then connected to electrical lead 24b.
The acoustic energy generated by such a transducer is a narrow beam normal to the front face plane of the transducer and is sometimes referred to as a boresight beam. The shape and size of the beam is dependent upon several factors which include overall size of the transducer, the frequency of excitation or reception, and the existence of shading induced by selectively suppressing the level of excitation or reception along the peripheral area of the transducer.
To generate/detect acoustic energy over a variety of azimuth and elevation angle combinations relative to the front face plane of a transducer, it is necessary to "steer" the boresight beam. In other words, the acoustically active portion of the front face plane must be controlled. To accomplish boresight beam steering, the entire transducer can be moved mechanically or the electrodes can be electronically steered by energizing the electrodes in accordance with a specific sequencing technique known in the art as phasing. Mechanical movement of the transducer involves slow, complex mechanisms. Electronic steering of transducer 10 requires each metal electrode 22a, 22b to have an individual electric lead attached thereto so that the outgoing beam can be steered along particular angles of azimuth and elevation relative to the front face plane or so that an incoming beam's angular resolution can be detected relative to the front face plane. However, implementing such individual connection is especially difficult and impractical when the transducer is designed for high-frequency operation. For example, a conventional high-frequency acoustic array of 400 electrodes (e.g., a 20.times.20 planar array) requires an electrical connection to each of the 400 electrodes of the array in order to have a steerable and controllable array. Thus, the front face plane of the array, i.e., the part that is emitting/receiving acoustic energy into/from the transmission medium, is a maze of 400 wires--one for each of the 400 individual electrodes. The conducting portion of each wire must be affixed to an individual electrode while the insulated portion of the wire must be routed to a connector or junction box. The wires can disrupt the acoustic beam being generated/received by the array and create an anisotropic volume above the array. Further, if such an array were built for a 250 kHz signal, the entire array would only measure about one inch across.
Another prior art approach to beam steering is disclosed in U.S. Pat. No. 4,202,050 where four sets of spirally stacked, linear arrays of individual piezoelectric crystals are used in conjunction with an electronic phasing signal generator/detector. However, operation of the device at high-frequency requires the use of arrays that are several feet in length. Such sizing is not practical for many devices requiring small acoustic transducers.
It is also often necessary to generate/detect acoustic energy over a variety of frequencies. For example, it may be necessary to determine the dependency of the beam's propagation distance upon the environment in which the acoustic energy is traveling. Typically, multiple single-frequency transducers are used to handle operation over a variety of frequencies. When using multiple ceramic transducers, e.g., multiples of transducer 100, the transducers must be arranged such that one transducer does not block the signal from any other transducer. This can be accomplished by varying the sizes of the transducers or spreading out the transducers. However, varying the sizes of the transducers always results in one or more frequencies having a lower sensitivity while spreading out the transducers requires additional space. Further, to date, multiple transducer designs lack symmetry about an axis of transmission/reception thereby complicating the signal processing associated therewith.